The “Mysterious” Origin of Brown Dwarfs

Hundreds of brown dwarfs (BDs) have been discovered in the last few years instellar clusters and among field stars. BDs are almost as numerous as hydrogenburning stars and so a theory of star formation should also explain theirorigin. The ``mystery'' of the origin of BDs is that their mass is two ordersof magnitude smaller than the average Jeans' mass in star--forming clouds, andyet they are so common. In this work we investigate the possibility thatgravitationally unstable protostellar cores of BD mass are formed directly bythe process of turbulent fragmentation. Supersonic turbulence in molecularclouds generates a complex density field with a very large density contrast. Asa result, a fraction of BD mass cores formed by the turbulent flow are denseenough to be gravitationally unstable. We find that with density, temperatureand rms Mach number typical of cluster--forming regions, turbulentfragmentation can account for the observed BD abundance.Comment: 11 pages, 3 figures, ApJ submitted Error in equation 1 has been corrected. Improved figure